This invention relates to a bend restrictor for optical fibre cable, and in particular to a bend restrictor for preventing a submarine optical fibre cable bending too much at a joint.
A submarine optical fibre cable has a plurality of optical fibres at its centre, the optical fibres being surrounded by a copper tube, which in turn is surrounded by two layers of helically-wound steel wires which are encapsulated in plastics material. The steel wires carry longitudinal cable forces, and the copper tube carries electrical power to components such as repeaters spaced along the cable. The optical fibres are subject to a bend restriction, namely that they should not bend with a radius of curvature of less than about one meter, and so the cable itself is also limited in this way. This gives rise to problems at cable joints (a cable joint is a connection from cable to cable) and at termination joints (a termination joint connects a cable to a repeater, via what is known as a pig-tail). For example, a cable joint has a steel sleeve surrounding the joint, and the cables leaving this sleeve tend to bend sharply when subject to transverse forces, particularly as the cable is being laid or recovered around the forward or aft sheaves of a cable ship or around a cable engine drum. Similar problems arise with termination joints.
A known bend limiter is constituted by a boot made of artificial rubber. The boot is tapered sleeve-like member, which is about one meter in length, the boot having a steel coupling ring moulded into its wider end. The coupling ring is externally threaded for connection to the steel sleeve of a cable joint. The wider end of the boot is about 5 to 6 inches (127 to 152 mm) in diameter, and the bore of the boot is about 1.5 inches (38 mm), the dimensions being chosen to take standard commercial cables of diameter between 1 and 1.25 inches (25 and 31 mm). The known type of boot tapers so that the boot has a varying rigidity along its length. The conical shape of the boot also acts as a smooth contour profile to allow the diameter discontinuity of the cable-to-joint interface to pass through or around ship's machinery with much reduced risk of fouling. The boot rigidity is arranged to vary from a very high value at its wider end (to match the rigidity of the steel sleeve of the cable joint) to a relatively low value at its narrower end (to match the rigidity of the cable).
The disadvantage of this type of bend limiter is that different cables have different structures, and so have different flexibilities. This implies that they will have different allowable minimum bend radii, so that different boots are required for different cables. This is particularly disadvantageous for ships which are used to repair submarine optical fibre cables, as these ships will need to carry a large inventory of boots to suit the many different types of submarine cable presently in use. This problem is compounded by the fact that the British, French, Japanese and Americans all use different cable structures and different joints.